In MRI imaging of chemical species such as fat and water within body tissue, the misregistration of fat in acquired magnetic resonance images can seriously degrade the quality of the image and obscure important pathology. As a diagnostic tool, such off-resonance effects, which may arise from field inhomogeneities, susceptibility or chemical shift, cause artifacts in the acquired images. The artifacts appear as positional shifts along the read direction in rectilinearly sampled acquisitions. Although various fat suppression methods have been conceived, such as using specialized radio-frequency (RF) excitation pulse sequences, such methods have various limitations. For example, inversion recovery pulse sequences and chemical shift selective excitation pulses have been attempted, but are limited by extended acquisition times and specific absorption rate (SAR) constraints when used as a diagnostic tool for human patients.
There have been various attempts at reducing scan times in obtaining reconstructed images of water-fat decomposition, but there is a further need for short scan times while obtaining high quality images. Dixon techniques have been utilized for water-fat decomposition in rectilinear sampling schemes. In a Dixon technique, water and fat images are generated by either addition or subtraction of the “in-phase” and “out-of-phase” data sets. With magnetic field inhomogeneity, such techniques may still lead to inaccurate decomposition of water and fat signals, resulting in a blurred image. In a two-point Dixon (2PD) technique, the two acquired images can be contaminated by off-resonance effects such that the water and fat images have significant components from the unwanted chemical species.
Modified Dixon techniques, such as the three-point Dixon (3PD) technique have also been developed to correct for magnetic field inhomogeneity off-resonance effects as well as susceptibility effects. The advantage of these multiple-point Dixon techniques over specialized RF excitation pulse-sequences or pulses lies in the ability to acquire water-fat separation even in the presence of magnetic field inhomogeneities, such as tissue-induced local inhomogeneity or applied magnetic field inhomogeneities in the field of view.
Although the multi-point Dixon techniques avoid SAR limitations, the typical Dixon implementation requires two acquisitions with different TE's to create a frequency map for off-resonance corrections. As an example, two acquisitions with different TE's are utilized where the Δ TE=2.2 MSEC @1.5 T. This characteristic Δ TE generates two complex data sets with 180° relative phase variation in the fat magnetization. The long acquisition times required for two separate acquisitions impose a significant limitation on the use of such techniques. It would therefore be desirable to increase the efficiency of the Dixon technique to reduce acquisition time required for accurate fat-water separation.